Any steel product produced by hot rolling needs descaling (which is a step placed before the secondary processing step such as drawing) to remove oxides which form on the surface of a steel billet (as a raw material) during heating and hot rolling. Descaling in practice includes mechanical descaling to remove scale physically or mechanically and pickling to remove scale chemically.
Incomplete descaling, with some scale remaining on the surface of the steel product, causes flaws at the time of drawing due to hard scale, which leads to a decreased die life or even a die breakage, resulting in reduced productivity.
Consequently, any steel product should be produced in such a way that it permits scale to be descaled easily by descaling, such as mechanical descaling (abbreviated as MD hereinafter) and pickling, that precedes the secondary processing step. Mechanical descaling is becoming more popular than before in view of recent environmental issue and cost reduction. Thus the ability of mechanical descaling to remove scale easily is a key to the production of steel products.
Mechanical descaling is physically accomplished by bending with rollers incorporated into the drawing line or by shot-blasting. However, mechanical descaling by bending is not effective if scale has scaled off before the drawing step, because in such a case, rust or thin tertiary scale occurs in scaled parts. The tertiary scale is very thin, hard magnetite scale, which cannot be removed easily by bending, and it breaks the die. Therefore, scale is required to have the property that it does not scale off before the drawing step but scales off easily at the time of bending or pickling.
Scale capable of being scaled off easily by MD or pickling should have a composition with a high content of FeO (wustite). Several ideas have so far been proposed to improve descalability by MD or pickling.
The object is achieved by winding the steel wire at a high temperature of 870 to 930° C. after rolling, thereby allowing easily scalable FeO to occur, and then cooling the steel wire rapidly, thereby suppressing the formation of hard-to-scale Fe3O4. (See Patent Document 1.) Unfortunately, winding alone at a high temperature is not enough for FeO to occur sufficiently in the case of hard steel wires containing much Si and C which tend to prevent the formation of FeO. Also, even in the case of soft steel wire, the foregoing method is not so effective in improving the MD performance because it merely keeps the steel wire at a high temperature for a very short time which is not enough for FeO to occur sufficiently.
Another method proposed so far consists of winding the steel wire at a temperature no higher than 800° C. and then cooling it at a cooling rate no lower than 0.5° C./sec until it cools from 600° C. to 400° C., thereby suppressing the formation of difficult-to-scale Fe2O4 (magnetite). (See Patent Document 2.) This method, however, does not form FeO sufficiently, as in the case of the method mentioned above, and hence it does not improve the descalability as intended.
Another method proposed so far is designed to uniformly cooling steel wires with an air blast directed into the hollow center of the coil of the wound steel wire, thereby controlling the composition and thickness of scale in a prescribed range over the entire length of the steel wire. (See Patent Document 3.) This method, however, is not so effective for hard steel wires containing much C and Si on which scale does not form easily.
All of the conventional methods mentioned above suffer the disadvantage that the scale layer in contact with steel is brittle FeO which is poor in adhesion after hot rolling. One way to improve scale adhesion effectively is by formation of fayalite (Fe2SiO4). However, no detailed investigation has been made from the standpoint of adhesion, and it poses a problem with the rust resistance of steel products.
There are additional methods proposed so far which are mainly designed to improve the mechanical properties of steel products by cooling. (See Patent Documents 4 and 5.) However, they are not satisfactory to give easily scalable scale.
Patent Document 1:
    Japanese Patent Laid-open No. Hei-4-293721Patent Document 2:    Japanese Patent Laid-open No. 2000-246322Patent Document 3:    Japanese Patent Laid-open No. 2005-118806Patent Document 4:    Japanese Patent Publication No. Hei-5-87566Patent Document 5:    Japanese Patent Laid-open No. 2004-10960